Command to transfer data from node state agent to memory bridge

ABSTRACT

A node comprises a first agent, a second agent, and a third agent, all coupled to an interconnect. The first agent is configured to initiate a transaction on the interconnect to transfer a coherency block to the second agent. The third agent is configured to transmit the coherency block on the interconnect during a data portion of the transaction instead of the first agent responsive to a state of the coherency block in the third agent. In some embodiments, the first agent may be designated to store the node state of a remote cache block, and the second agent may be responsible for internode coherency within the node.

[0001] This application claims benefit of priority to U.S. Provisional Patent Application Serial No. 60/380,740, filed May 15, 2002. This application is a continuation in part of U.S. patent application Ser. No. 10/270,028, filed on Oct. 11, 2002.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] This invention is related to coherent memory systems, including coherent distributed memory systems such as cache-coherent nonuniform memory access (CC-NUMA) memory systems.

[0004] 2. Description of the Related Art

[0005] Memory systems (including main memory and any caches in the system) are often designed to be coherent. That is, even though multiple copies of data from a given memory location may exist in the memory system, a read of that memory location returns the most recent data written to that memory location. The most recent data written to that memory location may be determined via an order of accesses to the memory location established according to the coherency mechanism. Typically, a coherent system may include one or more coherent agents and a memory controller coupled via an interconnect of some kind.

[0006] One mechanism for scaling coherent systems to larger numbers of coherent agents is a distributed memory system. In such a system, memory is distributed among various nodes (which may also include coherent agents), and the nodes are interconnected. A coherent agent in one node may access memory in another node. One class of techniques for maintaining coherency in a distributed memory system is referred to as cache-coherent, nonuniform memory access (CC-NUMA). In a CC-NUMA system, access to memory may have a varying latency (e.g. memory in the same node as an agent may be accessed more rapidly than memory in another node, and accesses to different nodes may have varying latencies as well), but coherency is maintained. Data from another node may be cached in a given node.

[0007] In some cases, it may be desirable to designate an agent in a node to maintain the state of data transferred to the node from another node. If that agent is to discard the data, a mechanism is desired to ensure that the data is deleted from other agents in the node. Additionally, the mechanism may be required to cause the most recently updated copy of the data, wherever it may reside within the node, to be returned to the node from which the data was read.

SUMMARY OF THE INVENTION

[0008] In one embodiment, a node comprises a first agent, a second agent, and a third agent, all coupled to an interconnect. The first agent is configured to initiate a transaction on the interconnect to transfer a coherency block to the second agent. The third agent is configured to transmit the coherency block on the interconnect during a data portion of the transaction instead of the first agent responsive to a state of the coherency block in the third agent. In some embodiments, the first agent may be designated to store the node state of a remote cache block, and the second agent may be responsible for internode coherency within the node.

[0009] In another embodiment, a node includes an interconnect and a cache coupled to the interconnect. The cache is configured to evict a cache block stored therein. The cache is configured to initiate a first transaction to transfer the cache block on the interconnect responsive to the cache block being a local cache block, and is configured to initiate a second transaction to transfer the cache block on the interconnect responsive to the cache block being a remote cache block. The second transaction is different from the first transaction.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] The following detailed description makes reference to the accompanying drawings, which are now briefly described.

[0011]FIG. 1 is a block diagram of one embodiment of a node.

[0012]FIG. 2 is a block diagram of one embodiment of a cache, a processor, and a memory bridge shown in FIG. 1 during one example of a write flush command.

[0013]FIG. 3 is a block diagram of one embodiment of a cache, a processor, and a memory bridge shown in FIG. 1 during another example of a write flush command.

[0014]FIG. 4 is a flowchart illustrating operation of one embodiment of the cache shown in FIGS. 1-3.

[0015]FIG. 5 is a table illustrating an exemplary set of coherency commands and a table illustrating an exemplary set of transactions according to one embodiment of the node shown in FIG. 1.

[0016]FIG. 6 is a block diagram of an address space supported by one embodiment of the node shown in FIG. 1.

[0017]FIG. 7 is a decision tree illustrating operation of one embodiment of a node for a read transaction on the interconnect within the node.

[0018]FIG. 8 is a decision tree illustrating operation of one embodiment of a node for a write transaction on the interconnect within the node.

[0019]FIG. 9 is a diagram illustrating operation of one embodiment of the memory bridge for remote coherency commands received by the memory bridge.

[0020]FIG. 10 is a block diagram of one embodiment of a computer accessible medium.

[0021] While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that the drawings and detailed description thereto are not intended to limit the invention to the particular form disclosed, but on the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the present invention as defined by the appended claims.

DETAILED DESCRIPTION OF EMBODIMENTS

[0022] Node Overview

[0023] Turning now to FIG. 1, a block diagram of one embodiment of a node 10 is shown. In the embodiment of FIG. 1, the node 10 includes one or more processors 12A-12N, a memory controller 14, a switch 18, a set of interface circuits 20A-20C, a memory bridge 32, and an L2 cache 36. The memory bridge 32 includes a remote line directory 34. The node 10 includes an interconnect 22 to which the processors 12A-12N, the memory controller 14, the L2 cache 36, the memory bridge 32, and the remote line directory 34 are coupled. The node 10 is coupled, through the memory controller 14, to a memory 24. The interface circuits 20A-20C each include a receive (Rx) circuit 26A-26C and a transmit (Tx) circuit 28A-28C. The node 10 is coupled to a set of interfaces 30A-30C through respective interface circuits 20A-20C. The interface circuits 20A-20C are coupled to the switch 18, which is further coupled to the memory bridge 32. A configuration register 38 is also illustrated in FIG. 1, which stores a node number (Node #) for the node 10. The configuration register 38 is coupled to the L2 cache 36, the memory controller 14, the memory bridge 32, and the interface circuits 20A-20C in the embodiment of FIG. 1. Additionally, the processors 12A-12N may be coupled to receive the node number from the configuration register 38.

[0024] The node 10 may support intranode coherency for transactions on the interconnect 22. Additionally, the node 10 may support internode coherency with other nodes (e.g. a CC-NUMA coherency, in one embodiment). Generally, as used herein, a memory bridge includes circuitry designed to handle internode coherency functions within a node. Particularly, in one embodiment, if a transaction on the interconnect 22 (e.g. a transaction issued by the processors 12A-12N) accesses a cache block that is remote to the node 10 (i.e. the cache block is part of the memory coupled to a different node) and the node 10 does not have sufficient ownership to perform the transaction, the memory bridge 32 may issue one or more coherency commands to the other nodes to obtain the ownership (and a copy of the cache block, in some cases). Similarly, if the transaction access a local cache block but one or more other nodes have a copy of the cache block, the memory bridge 32 may issue coherency commands to other nodes. Still further, the memory bridge 32 may receive coherency commands from other nodes, and may perform transactions on the interconnect 22 to effect the coherency commands.

[0025] In one embodiment, a node such as node 10 may have memory coupled thereto (e.g. memory 24). The node may be responsible for tracking the state, in other nodes, of each cache block from the memory in that node. A node is referred to as the “home node” for cache blocks from the memory assigned to that node. A node is referred to as a “remote node” for a cache block if the node is not the home node for that cache block. Similarly, a cache block is referred to as a local cache block in the home node for that cache block and as a remote cache block in other nodes.

[0026] Generally, a remote node may begin the coherency process by requesting a copy of a cache block from the home node of that cache block using a coherency command. The memory bridge 32 in the remote node, for example, may detect a transaction on the interconnect 22 that accesses the cache block and may detect that the remote node does not have sufficient ownership of the cache block to complete the transaction (e.g. it may not have a copy of the cache block at all, or may have a shared copy and may require exclusive ownership to complete the transaction). The memory bridge 32 in the remote node may generate and transmit the coherency command to the home node to obtain the copy or to obtain sufficient ownership. The memory bridge 32 in the home node may determine if any state changes in other nodes are to be performed to grant the requested ownership to the remote node, and may transmit coherency commands (e.g. probe commands) to effect the state changes. The memory bridge 32 in each node receiving the probe commands may effect the state changes and respond to the probe commands. Once the responses have been received, the memory bridge 32 in the home node may respond to the remote node (e.g. with a fill command including the cache block).

[0027] The remote line directory 34 may be used in the home node to track the state of the local cache blocks in the remote nodes. The remote line directory 34 is updated each time a cache block is transmitted to a remote node, the remote node returns the cache block to the home node, or the cache block is invalidated via probes. As used herein, the “state” of a cache block in a given node refers to an indication of the ownership that the given node has for the cache block according to the coherency protocol implemented by the nodes. Certain levels of ownership may permit no access, read-only access, or read-write access to the cache block. For example, in one embodiment, the modified, shared, and invalid states are supported in the internode coherency protocol. In the modified state, the node may read and write the cache block and the node is responsible for returning the block to the home node if evicted from the node. In the shared state, the node may read the cache block but not write the cache block without transmitting a coherency command to the home node to obtain modified state for the cache block. In the invalid state, the node may not read or write the cache block (i.e. the node does not have a valid copy of the cache block). Other embodiments may use other coherency protocols (e.g. the MESI protocol, which includes the modified, shared, and invalid states and an exclusive state in which the cache block has not yet been updated but the node is permitted to read and write the cache block, or the MOESI protocol which includes the modified, exclusive, shared, and invalid states and an owned state which indicates that there may be shared copies of the block but the copy in main memory is stale). In one embodiment, agents within the node may implement the MESI protocol for intranode coherency. Thus, the node may be viewed as having a state in the internode coherency and individual agents may have a state in the intranode coherency (consistent with the internode coherency state for the node containing the agent).

[0028] Coherency commands are transmitted and received on one of the interfaces 30A-30C by the corresponding interface circuit 20A-20C. The interface circuits 20A-20C receive coherency commands for transmission from the memory bridge 32 and transmit coherency commands received from the interfaces 30A-30C to the memory bridge 32 for processing, if the coherency commands require processing in the node 10. In some embodiments, a coherency command may be received that is passing through the node 10 to another node, and does not require processing in the node 10. The interface circuits 20A-20C may be configured to detect such commands and retransmit them (through another interface circuit 20A-20C) without involving the memory bridge 32.

[0029] In the illustrated embodiment, the interface circuits 20A-20C are coupled to the memory bridge 32 through the switch 18 (although in other embodiments, the interface circuits 20A-20C may have direct paths to the memory bridge 32). The switch 18 may selectively couple the interface circuits 20A-20C (and particularly the Rx circuits 26A-26C in the illustrated embodiment) to other interface circuits 20A-20C (and particularly the Tx circuits 28A-28C in the illustrated embodiment) or to the memory bridge 32 to transfer received coherency commands. The switch 18 may also selectively couple the memory bridge 32 to the interface circuits 20A-20C (and particularly to the Tx circuits 28A-28C in the illustrated embodiment) to transfer coherency commands generated by the memory bridge 32 from the memory bridge 32 to the interface circuits 20A-20C for transmission on the corresponding interface 30A-30C. The switch 18 may have request/grant interfaces to each of the interface circuits 20A-20C and the memory bridge 32 for requesting transfers and granting those transfers. The switch 18 may have an input path from each source (the Rx circuits 26A-26C and the memory bridge 32) and an output path to each destination (the Tx circuits 28A-28C and the memory bridge 32), and may couple a granted input path to a granted output path for transmission of a coherency command (or a portion thereof, if coherency commands are larger than one transfer through the switch 18). The couplings may then be changed to the next granted input path and granted output path. Multiple independent input path/output path grants may occur concurrently.

[0030] In one embodiment, the interfaces 30A-30C may support a set of virtual channels in which commands are transmitted. Each virtual channel is defined to flow independent of the other virtual channels, even though the virtual channels may share certain physical resources (e.g. the interface 30A-30C on which the commands are flowing). These virtual channels may be mapped to internal virtual channels (referred to as switch virtual channels herein). The switch 18 may be virtual-channel aware. That is, the switch 18 may grant a coupling between a source and a destination based not only on the ability of the source to transfer data and the destination to receive data, but also on the ability of the source to transfer data in a particular switch virtual channel and the destination to receive data on that switch virtual channel. Thus, requests from sources may indicate the destination and the virtual channel on which data is to be transferred, and requests from destinations may indicate the virtual channel on which data may be received.

[0031] Generally speaking, a node may include one or more coherent agents (dotted enclosure 16 in FIG. 1). In the embodiment of FIG. 1, the processors 12A-12N, the L2 cache 36, and the memory controller 14 may be examples of coherent agents 16. Additionally, the memory bridge 32 may be a coherent agent (on behalf of other nodes). However, other embodiments may include other coherent agents as well, such as a bridge to one or more I/O interface circuits, or the I/O interface circuits themselves. Generally, an agent includes any circuit which participates in transactions on an interconnect. A coherent agent is an agent that is capable of performing coherent transactions and operating in a coherent fashion with regard to transactions. A transaction is a communication on an interconnect. The transaction is sourced by one agent on the interconnect, and may have one or more agents as a target of the transaction. Read transactions specify a transfer of data from a target to the source, while write transactions specify a transfer of data from the source to the target. Other transactions may be used to communicate between agents without transfer of data, in some embodiments.

[0032] Each of the interface circuits 20A-20C are configured to receive and transmit on the respective interfaces 30A-30C to which they are connected. The Rx circuits 26A-26C handle the receiving of communications from the interfaces 30A-30C, and the Tx circuits 28A-28C handle the transmitting of communications on the interfaces 30A-30C.

[0033] Each of the interfaces 30A-30C used for coherent communications are defined to be capable of transmitting and receiving coherency commands. Particularly, in the embodiment of FIG. 1, those interfaces 30A-30C may be defined to receive/transmit coherency commands to and from the node 10 from other nodes. Additionally, other types of commands may be carried. In one embodiment, each interface 30A-30C may be a HyperTransport™ (HT) interface, including an extension to the HT interface to include coherency commands (HTcc). Additionally, in some embodiments, an extension to the HyperTransport interface to carry packet data (Packet over HyperTransport, or PoHT) may be supported. As used herein, coherency commands include any communications between nodes that are used to maintain coherency between nodes. The commands may include read or write requests initiated by a node to fetch or update a cache block belonging to another node, probes to invalidate cached copies of cache blocks in remote nodes (and possibly to return a modified copy of the cache block to the home node), responses to probe commands, fills which transfer data, etc.

[0034] In some embodiments, one or more of the interface circuits 20A-20C may not be used for coherency management and may be defined as packet interfaces. Such interfaces 30A-30C may be HT interfaces. Alternative, such interfaces 30A-30C may be system packet interfaces (SPI) according to any level of the SPI specification set forth by the Optical Internetworking Forum (e.g. level 3, level 4, or level 5). In one particular embodiment, the interfaces may be SPI-4 phase 2 interfaces. In the illustrated embodiment, each interface circuit 20A-20C may be configurable to communicate on either the SPI-4 interface or the HT interface. Each interface circuit 20A-20C may be individually programmable, permitting various combinations of the HT and SPI-4 interfaces as interfaces 30A-30C. The programming may be performed in any fashion (e.g. sampling certain signals during reset, shifting values into configuration registers (not shown) during reset, programming the interfaces with configuration space commands after reset, pins that are tied up or down externally to indicate the desired programming, etc.). Other embodiments may employ any interface capable of carrying packet data (e.g. the Media Independent Interface (MII) or the Gigabit MII (GMII) interfaces, X.25, Frame Relay, Asynchronous Transfer Mode (ATM), etc.). The packet interfaces may carry packet data directly (e.g. transmitting the packet data with various control information indicating the start of packet, end of packet, etc.) or indirectly (e.g. transmitting the packet data as a payload of a command, such as PoHT).

[0035] In embodiments which also support packet traffic, the node 10 may also include a packet direct memory access (DMA) circuit configured to transfer packets to and from the memory 24 on behalf of the interface circuits 20A-20C. The switch 18 may be used to transmit packet data from the interface circuits 20A-20C to the packet DMA circuit and from the packet DMA circuit to the interface circuits 20A-20C. Additionally, packets may be routed from an Rx circuit 26A-26C to a Tx circuit 28A-28C through the switch 18, in some embodiments.

[0036] The processors 12A-12N may be designed to any instruction set architecture, and may execute programs written to that instruction set architecture. Exemplary instruction set architectures may include the MIPS instruction set architecture (including the MIPS-3D and MIPS MDMX application specific extensions), the IA-32 or IA-64 instruction set architectures developed by Intel Corp., the PowerPC instruction set architecture, the Alpha instruction set architecture, the ARM instruction set architecture, or any other instruction set architecture. The node 10 may include any number of processors (e.g. as few as one processor, two processors, four processors, etc.).

[0037] The L2 cache 36 may be any type and capacity of cache memory, employing any organization (e.g. set associative, direct mapped, fully associative, etc.). In one embodiment, the L2 cache 36 may be an 8 way, set associative, 1 MB cache. The L2 cache 36 is referred to as L2 herein because the processors 12A-12N may include internal (L1) caches. In other embodiments the L2 cache 36 may be an L1 cache, an L3 cache, or any other level as desired.

[0038] The memory controller 14 is configured to access the memory 24 in response to read and write transactions received on the interconnect 22. The memory controller 14 may receive a hit signal from the L2 cache, and if a hit is detected in the L2 cache for a given read/write transaction, the memory controller 14 may not respond to that transaction. The memory controller 14 may be designed to access any of a variety of types of memory. For example, the memory controller 14 may be designed for synchronous dynamic random access memory (SDRAM), and more particularly double data rate (DDR) SDRAM. Alternatively, the memory controller 16 may be designed for DRAM, DDR synchronous graphics RAM (SGRAM), DDR fast cycle RAM (FCRAM), DDR-II SDRAM, Rambus DRAM (RDRAM), SRAM, or any other suitable memory device or combinations of the above mentioned memory devices.

[0039] The interconnect 22 may be any form of communication medium between the devices coupled to the interconnect. For example, in various embodiments, the interconnect 22 may include shared buses, crossbar connections, point-to-point connections in a ring, star, or any other topology, meshes, cubes, etc. The interconnect 22 may also include storage, in some embodiments. In one particular embodiment, the interconnect 22 may comprise a bus. The bus may be a split transaction bus, in one embodiment (i.e. having separate address and data phases). The data phases of various transactions on the bus may proceed out of order with the address phases. The bus may also support coherency and thus may include a response phase to transmit coherency response information. The bus may employ a distributed arbitration scheme, in one embodiment. In one embodiment, the bus may be pipelined. The bus may employ any suitable signaling technique. For example, in one embodiment, differential signaling may be used for high speed signal transmission. Other embodiments may employ any other signaling technique (e.g. TTL, CMOS, GTL, HSTL, etc.). Other embodiments may employ non-split transaction buses arbitrated with a single arbitration for address and data and/or a split transaction bus in which the data bus is not explicitly arbitrated. Either a central arbitration scheme or a distributed arbitration scheme may be used, according to design choice. Furthermore, the bus may not be pipelined, if desired.

[0040] Various embodiments of the node 10 may include additional circuitry, not shown in FIG. 1. For example, the node 10 may include various I/O devices and/or interfaces. Exemplary I/O may include one or more PCI interfaces, one or more serial interfaces, Personal Computer Memory Card International Association (PCMCIA) interfaces, etc. Such interfaces may be directly coupled to the interconnect 22 or may be coupled through one or more I/O bridge circuits.

[0041] In one embodiment, the node 10 (and more particularly the processors 12A-12N, the memory controller 14, the L2 cache 36, the interface circuits 20A-20C, the memory bridge 32 including the remote line directory 34, the switch 18, the configuration register 38, and the interconnect 22) may be integrated onto a single integrated circuit as a system on a chip configuration. The additional circuitry mentioned above may also be integrated. Alternatively, other embodiments may implement one or more of the devices as separate integrated circuits. In another configuration, the memory 24 may be integrated as well. Alternatively, one or more of the components may be implemented as separate integrated circuits, or all components may be separate integrated circuits, as desired. Any level of integration may be used.

[0042] It is noted that, while three interface circuits 20A-20C are illustrated in FIG. 1, one or more interface circuits may be implemented in various embodiments. As used herein, an interface circuit includes any circuitry configured to communicate on an interface according to the protocol defined for the interface. The interface circuit may include receive circuitry configured to receive communications on the interface and transmit the received communications to other circuitry internal to the system that includes the interface circuit. The interface circuit may also include transmit circuitry configured to receive communications from the other circuitry internal to the system and configured to transmit the communications on the interface.

[0043] It is noted that the discussion herein may describe cache blocks and maintaining coherency on a cache block granularity (that is, each cache block has a coherency state that applies to the entire cache block as a unit). Other embodiments may maintain coherency on a different granularity than a cache block, which may be referred to as a coherency block. A coherency block may be smaller than a cache line, a cache line, or larger than a cache line, as desired. The discussion herein of cache blocks and maintaining coherency therefor applies equally to coherency blocks of any size.

[0044] Write Flush Transaction

[0045] The node 10 may cache one or more remote cache blocks, and may have a state in the node 10 for the cache block (the “node state”). The node state may be the state in which the remote cache block is provided to the node 10 (e.g. shared or modified) that is tracked by the home node of the remote cache block. If the node state in the node 10 is modified, the node 10 returns the cache block to the home node when the node 10 evicts the cache block. In such cases, the home node may update the state for the remote cache block to indicate that the node 10 no longer has a copy. Accordingly, when evicting a remote cache block with a modified node state from the node 10, the node 10 ensures that any copies of the remote cache block in the node 10 are invalidated.

[0046] The node 10 may include a transaction for transferring an evicted remote cache block to the memory bridge 32 for return to its home node. The transaction may be defined to transfer the cache block addressed by the transaction to the agent responsible for internode coherency (e.g. the memory bridge 32, in the embodiment of FIG. 1). The transaction (referred to as a write flush transaction, or WrFlush transaction, herein for convenience) may be used for evicting remote cache blocks, while other transactions (transactions with different command encodings on the interconnect 22) may be used to coherently or non-coherently transfer data among the agents (including the memory bridge 32, for remote transactions). The WrFlush transaction may be received by other coherent agents within the node 10, and the coherent agents may invalidate the remote cache block if stored therein in response to the WrFlush transaction. Additionally, if a coherent agent that did not initiate the WrFlush transaction has a modified copy of the remote cache block, that coherent agent may transmit the cache block during the data portion of the WrFlush transaction instead of the initiating agent. The coherent agent may signal the initiating agent (e.g. during a response portion of the transaction) that the coherent agent is to transfer the data.

[0047] There may be various coherent agents in the node 10 (e.g. the L2 cache 36, the processors 12A-12N, etc.), one or more of which may have a copy of a given remote cache block. Multiple copies may exist when various agents share the remote cache block according to the intranode coherency scheme employed on the interconnect 22. For example, in one embodiment, a MESI protocol may be employed, and may be enforced by the coherent agents snooping transactions on the interconnect 22. Other embodiments may employ a MOESI coherency protocol or any other protocol. Thus, when a remote cache block is being evicted from the node 10, the ownership of the remote cache block may be not be determined by the initiating agent prior to initiating the WrFlush transaction. The WrFlush transaction permits an exclusive owner (according to the intranode coherent protocol) to transfer the cache block, or the initiating agent transfers the cache block if no other agent is the exclusive owner.

[0048] In one embodiment, the node 10 may include an agent designated for retaining the node state (that is, the state that the home node is tracking for the node 10) of a remote cache block. For example, the L2 cache 36 may be the designated agent (although any agent may be designated in other embodiments). When the node 10 fetches a remote cache block into the node 10, the L2 cache 36 may allocate a storage location to store the remote cache block. The state of the cache block is stored in the L2 cache 36 as well.

[0049] The L2 cache 36 may permit other coherent agents to access the remote cache block and to cache the remote cache block in any state that is consistent with the node state (e.g. any state that is less permissive than the node state or has the same permissiveness as the node state, where read access is less permissive than write access or read/write access). For example, if the node state is modified, any state may be maintained by a caching node. If the node state is shared, only the shared or invalid states are consistent with the node state. Since other coherent agents may have a copy of a remote cache block, if the L2 cache 36 evicts the remote cache block, the other coherent agents may be forced to evict the remote cache block as well. In some embodiments, the L2 cache 36 may force the eviction if the node state is modified, but not if the node state is shared. However, since the L2 cache 36 allows other coherent agents to cache the remote cache block in the modified state (and thus permits these coherent agents to modify the remote cache block), another coherent agent may have a copy of the remote cache block which is more up to date than the L2 cache's copy.

[0050] In such an embodiment, the L2 cache 36 may use the WrFlush transaction to evict remote cache blocks having a modified node state. The L2 cache 36 may initiate the WrFlush transaction on the interconnect 22. The coherent agents may snoop the WrFlush transaction, and may invalidate any copies of the remote cache block in the coherent agents in response. Each coherent agent may signal a snoop response to the L2 cache 36 during a response portion of the WrFlush transaction. If a coherent agent has a modified copy of the cache block, that coherent agent may signal exclusive in the response portion. That coherent agent may then become responsible for transmitting the remote cache block in the data portion of the WrFlush transaction. If no coherent agent signals exclusive, the L2 cache 36 transmits the remote cache block in the data portion of the WrFlush transaction. In either case, the memory bridge 32 receives the WrFlush transaction and the remote cache block for transfer to the home node of the remote cache block.

[0051] In one embodiment, the L2 cache 36 may use the WrFlush transaction to evict remote cache blocks to the memory bridge 32 and may use a different transaction (e.g. a write (Wr) transaction) to evict local cache blocks. The memory controller 14 may receive the Wr transactions and may update the memory 24 with the local cache blocks received from the L2 cache 36. In some embodiments, the coherent agents do not snoop the Wr transaction. In some embodiments, the memory controller 14 may not process the WrFlush transaction. For example, the memory controller 14 may detect the WrFlush transaction and ignore the transaction, or may detect that the transaction address a remote cache block and ignore the transaction.

[0052] In some embodiments, a coherent agent may not necessarily determine if it has a modified copy of the cache block to signal exclusive in the response portion. For example, the processors 12A-12N may maintain a set of snoop tags for the data caches, used for snooping purposes. The snoop tags may track the state of the tags in the data cache, except for the modified state. That is, the snoop tags may have invalid, shared, or exclusive states (where exclusive or modified in the data cache is represented by the exclusive state). In such embodiments, a processors 12A-12N may transfer the cache block for the WrFlush transaction if the processor has the cache block in either the exclusive or the modified state.

[0053] In one embodiment, the WrFlush transaction may be decoded by the snooping agents (e.g. the processors 12A-12N) as an exclusive read transaction. Thus, the WrFlush transaction may have the same effect on the snooping agent as an exclusive read transaction would have.

[0054] It is noted that, while the WrFlush transaction is described above for transmitting a remote cache block to the memory bridge 32, the WrFlush transaction may have other uses as well. Generally, the WrFlush transaction may be initiated by an initiating agent to transfer a cache block to a second agent, where the cache block may be transmitted by either the initiating agent or a third agent dependent upon the state of the cache block in the third agent.

[0055]FIGS. 2 and 3 are block diagrams illustrating the L2 cache 36, the processor 12A, and the memory bridge 32 for example operation of the WrFlush transaction according to one embodiment of the node 10. FIG. 2 is an example of the WrFlush transaction in which the processor 12A has the remote cache block in the shared state, and FIG. 3 is an example of the WrFlush transaction in which the processor 12A has the remote cache block in the modified state. The remote cache block is addressed by the address “A1” in the examples.

[0056] In the example of FIG. 2, the L2 cache 36 has the remote cache block addressed by the address A1 in the modified state. In other words, the node state of the remote cache block is modified. Additionally, the processor 12A has a copy of the remote cache block in the shared state. The L2 cache 36 evicts the cache block and, detecting that the cache block is a remote cache block, initiates a WrFlush transaction to transfer the remote cache block to the memory bridge 32 (arrow 40). Additionally, the processor 12A snoops the WrFlush transaction, and detects that the remote cache block is cached in the shared state. The processor 12A invalidates the remote cache block in its cache. During a response portion of the WrFlush transaction, the processor 12A transmits a shared response to the L2 cache 36 (arrow 42). Since there is not an exclusive response, the L2 cache 36 transmits the remote cache block during the data portion of the transaction (arrow 44). The memory bridge 32 receives the remote cache block, and subsequently transmits the remote cache block to the home node (not shown in FIG. 2).

[0057] In the example of FIG. 3, the L2 cache 36 has the remote cache block addressed by the address A1 in the modified state. In other words, the node state of the remote cache block is modified. Additionally, the processor 12A has a copy of the remote cache block in the modified state. The processor 12A's copy is more up to date than the L2 cache's copy, and thus should be transmitted to the memory bridge 32 for return to the home node. The L2 cache 36 evicts the cache block and, detecting that the cache block is a remote cache block, initiates a WrFlush transaction to transfer the remote cache block to the memory bridge 32 (arrow 50). Additionally, the processor 12A snoops the WrFlush transaction, and detects that the remote cache block is cached in the modified state. The processor 12A invalidates the remote cache block in its cache. During a response portion of the WrFlush transaction, the processor 12A transmits an exclusive response to the L2 cache 36 (arrow 52). Since there is an exclusive response, the L2 cache 36 inhibits transmission of the remote cache block during the data portion of the transaction. Instead, the processor 12A transmits the remote cache block during the data portion (arrow 54). The memory bridge 32 receives the remote cache block, and subsequently transmits the remote cache block to the home node (not shown in FIG. 3).

[0058] It is noted that, in various embodiments, there may be additional coherent agents coupled to the interconnect 22 (e.g. additional processors 12A-12N, I/O bridges, I/O interfaces, etc.). If any coherent agent responds exclusive, that coherent agent may transmit the remote cache block in the data portion of the WrFlush transaction (similar to FIG. 3). If no coherent agent responds exclusive, the data portion of the WrFlush transaction may proceed as in FIG. 2. As mentioned above, in some embodiments, the processors 12A-12N (and/or other coherent snooping agents) may not determine whether or not a remote cache block is exclusive or modified to respond in the response phase. In such embodiments, the snooping agent may respond exclusive if the remote cache block is in the exclusive state or the modified state in the snooping agent, and may signal with the data transfer whether or not the remote cache block is modified.

[0059] It is noted that, in some embodiments, a given coherent agent may invalidate a cache block in its cache at a delayed time with regard to the snoop that causes the block to be invalidated. For example, the snoop may be queued for later invalidation, and the queue may be checked in parallel with a cache access to ensure the invalidated cache block is not used.

[0060] Generally, a “snoop” may include receiving a transaction initiated on the interconnect 22, and checking for the state of the cache block addressed by the transaction in response to the transaction. The result of the snoop may be signaled (e.g. during a response portion of the transaction), and a state change may be effected in the snooping agent for the cache block addressed by the transaction (if appropriate based on the transaction and the current state, according to the coherency protocol). A copy of a cache block may be modified if the copy has been changed by the caching agent from the copy that was supplied to that caching agent.

[0061] It is noted that, in some embodiments, the L2 cache 36 may be programmable to reserve one or more locations for storing remote cache blocks. Such embodiments may inhibit selecting the reserved locations for replacement to store local cache blocks that miss in the L2 cache 36. For example, set associative embodiments may be programmable to reserve one or more ways for remote cache blocks.

[0062] It is noted that portions of the WrFlush transaction are referred to above (e.g. the response portion and the data portion). Depending on the nature of the interconnect 22, the portions may occur in various fashions. For example, a packet-based interconnect may include a request packet to initiate the transaction (including the address A1), one or more response packets indicating the response (which may be included in the response portion) and a data packet (which may be included in the data portion). If the interconnect is a bus, the response portion and data portion may be phases on the bus (e.g. a split transaction bus may include an address phase on the address bus, a response phase on the response lines, and a data phase on the data bus).

[0063] Turning next to FIG. 4, a flowchart is shown illustrating operation of one embodiment of the L2 cache 36 in response to determining that a cache block is to be evicted (e.g. in response to an L2 cache miss for a different cache block). The blocks in FIG. 4 are illustrated in a particular order for ease of understanding, but any order may be used. Furthermore, blocks may be performed in parallel by combinatorial logic circuitry in the L2 cache 36. Still further, blocks may be pipelined over multiple clock cycles and/or the flowchart of FIG. 4 may represent operation over a multiple clock cycles.

[0064] The L2 cache 36 may determine if the evicted cache block is in the modified state (decision block 60). If the evicted cache block is not modified, in this embodiment, the L2 cache 36 need not perform a transaction on the interconnect 22 irrespective of whether the evicted cache block is local or remote. Thus, if the evicted cache block is not in the modified state (decision block 60—“no” leg), the L2 cache 36 may drop the evicted cache block (block 62).

[0065] If the evicted cache block is modified (decision block 60—“yes” leg), the L2 cache block may determine if the cache block is a remote cache block (decision block 64). In one embodiment, a remote cache block may be detectable by its address. For example, one embodiment described below with regard to FIGS. 5-9 maps addresses to nodes based on the most significant bits of address and the node number in the node. Other embodiments may detect remote cache blocks in other fashions (e.g. the L2 cache 36 may store an indication for each cache block, indicating if it is local or remote). If the cache block is local (decision block 64—“no” leg), the L2 cache 36 initiates a write transaction (Wr transaction) on the interconnect 22 to transfer the local cache block to the memory controller 14 (block 66). This transaction may not be snooped, and thus the L2 cache 36 may transmit the cache block during the data portion of the Wr transaction.

[0066] If the cache block is remote (decision block 64—“yes” leg), the L2 cache 36 initiates a WrFlush transaction on the interconnect 22 (block 68). The L2 cache 36 may wait for the response portion of the WrFlush transaction and, if an exclusive response is received (decision block 70—“yes” leg), the L2 cache 36 may inhibit transmitting the cache block during the data portion. On the other hand, if an exclusive response is not received (decision block 70—“no” leg), the L2 cache 36 transmits the cache block to the memory bridge 32 during the data portion of the transaction (block 72).

[0067] Additional CC-NUMA Details, One Embodiment

[0068] FIGS. 5-9 illustrate additional details regarding one exemplary embodiment of a CC-NUMA protocol that may be employed by one embodiment of the node 10. The embodiment of FIGS. 5-9 is merely exemplary. Numerous other implementations of CC-NUMA protocols or other distributed memory system protocols may be used in other embodiments.

[0069] Turning next to FIG. 5, a table 142 is shown illustrating an exemplary set of transactions supported by one embodiment of the interconnect 22 and a table 144 is shown illustrating an exemplary set of coherency commands supported by one embodiment of the interfaces 30. Other embodiments including subsets, supersets, or alternative sets of commands may be used.

[0070] The transactions illustrated in the table 142 will next be described. An agent in the node 10 may read a cache block (either remote or local) using the read shared (RdShd) or read exclusive (RdExc) transactions on the interconnect 22. The RdShd transaction is used to request a shared copy of the cache block, and the RdExc transaction is used to request an exclusive copy of the cache block. If the RdShd transaction is used, and no other agent reports having a copy of the cache block during the response phase of the transaction (except for the L2 cache 36 and/or the memory controller 14), the agent may take the cache block in the exclusive state. In response to the RdExc transaction, other agents in the node invalidate their copies of the cache block (if any). Additionally, an exclusive (or modified) owner of the cache block may supply the data for the transaction in the data phase. Other embodiments may employ other mechanisms (e.g. a retry on the interconnect 22) to ensure the transfer of a modified cache block.

[0071] The write transaction (Wr) and the write invalidate transaction (WrInv) may be used by an agent to write a cache block to memory. The Wr transaction may be used by an owner having the modified state for the block, since no other copies of the block need to be invalidated. The WrInv transaction may be used by an agent that does not have exclusive ownership of the block (the agent may even have the invalid state for the block). The WrInv transaction causes other agents to invalidate any copies of the block, including modified copies. The WrInv transaction may be used by an agent that is writing the entire cache block. For example, a DMA that is writing the entire cache block with new data may use the transaction to avoid a read transaction followed by a write transaction.

[0072] The RdKill and RdInv transactions may be used by the memory bridge 32 in response to probes received by the node 10 from other nodes. The RdKill and RdInv transactions cause the initiator (the memory bridge 32) to acquire exclusive access to the cache block and cause any cache agents to invalidate their copies (transferring data to the initiator similar to the RdShd and RdExc transactions). In one embodiment, the RdKill transaction also cancels a reservation established by the load-linked instruction in the MIPS instruction set, while the RdInv transaction does not. In other embodiments, a single transaction may be used for probes. In still other embodiments, there may be a probe-generated transaction that invalidates agent copies of the cache block (similar to the RdKill and RdInv transactions) and another probe-generated transaction that permits agents to retain shared copies of the cache block.

[0073] The WrFlush transaction is a write transaction which may be initiated by an agent and another agent may have an exclusive or modified copy of the block. The other agent provides the data for the WrFlush transaction, or the initiating agent provides the data if no other agent has an exclusive or modified copy of the block. The WrFlush transaction may be used, in one embodiment as described above by the L2 cache 36.

[0074] The Nop transaction is a no-operation transaction. The Nop may be used if an agent is granted use of the interconnect 22 (e.g. the address bus, in embodiments in which the interconnect 22 is a split transaction bus) and the agent determines that it no longer has a transaction to run on the interconnect 22.

[0075] The commands illustrated in the table 144 will next be described. In the table 144, the command is shown as well as the virtual channel in which the command travels on the interfaces 30. The virtual channels may include, in the illustrated embodiment: the coherent read (CRd) virtual channel; the probe (Probe) virtual channel; the acknowledge (Ack) virtual channel; and coherent fill (CFill) virtual channel. The CRd, Probe, Ack, and CFill virtual channels are defined for the HTcc commands. There may be additional virtual channels for the standard HT commands (e.g. non-posted command (NPC) virtual channel, the posted command (PC) virtual channel, and the response (RSP) virtual channel).

[0076] The cRdShd or cRdExc commands may be issued by the memory bridge 32 in response to a RdShd or RdExc transactions on the interconnect 22, respectively, to read a remote cache block not stored in the node (or, in the case of RdExc, the block may be stored in the node but in the shared state). If the cache block is stored in the node (with exclusive ownership, in the case of the RdExc transaction), the read is completed on the interconnect 22 without any coherency command transmission by the memory bridge 32.

[0077] The Flush and Kill commands are probe commands for this embodiment. The memory bridge 32 at the home node of a cache block may issue probe commands in response to a cRdShd or cRdExc command. The memory bridge 32 at the home node of the cache block may also issue a probe command in response to a transaction for a local cache block, if one or more remote nodes has a copy of the cache block. The Flush command is used to request that a remote modified owner of a cache block return the cache block to the home node (and invalidate the cache block in the remote modified owner). The Kill command is used to request that a remote owner invalidate the cache block. In other embodiments, additional probe commands may be supported for other state change requests (e.g. allowing remote owners to retain a shared copy of the cache block).

[0078] The probe commands are responded to (after effecting the state changes requested by the probe commands) using either the Kill_Ack or WB commands. The Kill_Ack command is an acknowledgement that a Kill command has been processed by a receiving node. The WB command is a write back of the cache block, and is transmitted in response to the Flush command. The WB command may also be used by a node to write back a remote cache block that is being evicted from the node.

[0079] The Fill command is the command to transfer data to a remote node that has transmitted a read command (cRdExc or cRdShd) to the home node. The Fill command is issued by the memory bridge 32 in the home node after the probes (if any) for a cache block have completed.

[0080] Turning next to FIG. 6, a block diagram illustrating one embodiment of an address space implemented by one embodiment of the node 10 is shown. Addresses shown in FIG. 6 are illustrated as hexadecimal digits, with an under bar (“_”) separating groups of four digits. Thus, in the embodiment illustrated in FIG. 6, 40 bits of address are supported. In other embodiments, more or fewer address bits may be supported.

[0081] In the embodiment of FIG. 6, the address space between 00_(—)0000_(—)0000 and 0F_FFFF_FFFF is treated as local address space. Transactions generated by agents in the local address space do not generate coherency commands to other nodes, although coherency may be enforced within the node 10 for these addresses. That is, the local address space is not maintained coherent with other nodes. Various portions of the local address space may be memory mapped to I/O devices, HT, etc. as desired.

[0082] The address space between 40_(—)0000_(—)0000 and EF_FFFF_FFFF is the remote coherent space 148. That is, the address space between 40_(—)0000_(—)0000 and EF_FFFF_FFFF is maintained coherent between the nodes. Each node is assigned a portion of the remote coherent space, and that node is the home node for the portion. As shown in FIG. 1, each node is programmable with a node number. The node number is equal to the most significant nibble (4 bits) of the addresses for which that node is the home node, in this embodiment. Thus, the node numbers may range from 4 to E in the embodiment shown. Other embodiments may support more or fewer node numbers, as desired. In the illustrated embodiment, each node is assigned a 64 Gigabyte (GB) portion of the memory space for which it is the home node. The size of the portion assigned to each node may be varied in other embodiments (e.g. based on the address size or other factors).

[0083] For a given coherent node, there is an aliasing between the remote coherent space for which that node is the home node and the local address space of that node. That is, corresponding addresses in the local address space and the portion of the remote coherent space for which the node is the home node access the same memory locations in the memory 24 of the node (or are memory mapped to the same I/O devices or interfaces, etc.). For example, the node having node number 5 aliases the address space 50_(—)0000_(—)0000 through 5F_FFFF_FFFF to 00_(—)0000_(—)0000 through 0F_FFFF_FFFF respectively (arrow 146). Internode coherent accesses to the memory 24 at the node 10 use the node-numbered address space (e.g. 50_(—)0000_(—)0000 to 5F_FFFF_FFFF, if the node number programmed into node 10 is 5) to access cache blocks in the memory 24. That is agents in other nodes and agents within the node that are coherently accessing cache blocks in the memory use the remote coherent space, while access in the local address space are not maintained coherent with other nodes (even though the same cache block may be accessed). Thus the addresses are aliased, but not maintained coherent, in this embodiment. In other embodiments, the addresses in the remote coherent space and the corresponding addresses in the local address space may be maintained coherent.

[0084] A cache block is referred to as local in a node if the cache block is part of the memory assigned to the node (as mentioned above). Thus, the cache block may be local if it is accessed from the local address space or the remote coherent space, as long as the address is in the range for which the node is the home node. Similarly, a transaction on the interconnect 22 that accesses a local cache block may be referred to as a local transaction or local access. A transaction on the interconnect 22 that accesses a remote cache block (via the remote coherent address space outside of the portion for which the node is the home node) may be referred to as a remote transaction or a remote access.

[0085] The address space between 10_(—)0000_(—)0000 and 3F_FFFF_FFFF may be used for additional HT transactions (e.g. standard HT transactions) in the illustrated embodiment. Additionally, the address space between F0_(—)0000_(—)0000 and FF_FFFF_FFFF may be reserved in the illustrated embodiment.

[0086] It is noted that, while the most significant nibble of the address defines which node is being accessed, other embodiments may use any other portion of the address to identify the node. Furthermore, other information in the transaction may be used to identify remote versus local transactions, in other embodiments (e.g. command type, control information transmitted in the transaction, etc.).

[0087] Turning next to FIG. 7, a decision tree for a read transaction to a memory space address on the interconnect 22 of a node 10 is shown for one embodiment. The decision tree may illustrate operation of the node 10 for the read transaction for different conditions of the transaction, the state of the cache block accessed by the transaction, etc. The read transaction may, in one embodiment, include the RdShd, RdExc, RdKill, and RdInv transactions shown in the table 142 of FIG. 5. Each dot on the lines within the decision tree represents a divergence point of one or more limbs of the tree, which are labeled with the corresponding conditions. Where multiple limbs emerge from a dot, taking one limb also implies that the conditions for the other limbs are not met. In FIG. 7, the exclamation point (“!”) is used to indicate a logical NOT. Not shown in FIG. 7 is the state transition made by each coherent agent which is caching a copy of the cache block for the read transaction. If the read transaction is RdShd, the coherent agent may retain a copy of the cache block in the shared state. Otherwise, the coherent agent invalidates its copy of the cache block.

[0088] The transaction may be either local or remote, as mentioned above. For local transactions, if the transaction is uncacheable, then a read from the memory 24 is performed (reference numeral 150). In one embodiment, the transaction may include an indication of whether or not the transaction is cacheable. If the transaction is uncacheable, it is treated as a non-coherent transaction in the present embodiment.

[0089] If the local transaction is cacheable, the operation of the node 10 is dependent on the response provided during the response phase of the transaction. In one embodiment, each coherent agent responds with the state of the cache block in that agent. For example, each coherent agent may have an associated shared (SHD) and exclusive (EXC) signal. The agent may signal invalid state by deasserting both the SHD and EXC signals. The agent may signal shared state by asserting the SHD signal and deasserting the EXC signal. The agent may signal exclusive state (or modified state) by asserting the EXC signal and deasserting the SHD signal. The exclusive and modified states may be treated the same in the response phase in this embodiment, and the exclusive/modified owner may provide the data. The exclusive/modified owner may provide, concurrent with the data, an indication of whether the state is exclusive or modified. While each agent may have its own SHD and EXC signals in this embodiment (and the initiating agent may receive the signals from each other agent), in other embodiments a shared SHD and EXC signal may be used by all agents.

[0090] If both the SHD and EXC responses are received for the local transaction, an error has occurred (reference numeral 152). The memory controller may return a fatal error indication for the read transaction, in one embodiment. If the response is exclusive (SHD deasserted, EXC asserted) the exclusive owner provides the data for the read transaction on the interconnect 22 (reference numeral 154). If the exclusive owner is the memory bridge 32 (as recorded in the remote line directory 34), then a remote node has the cache block in the modified state. The memory bridge 32 issues a probe (Flush command) to retrieve the cache block from that remote node. The memory bridge 32 may supply the cache block returned from the remote node as the data for the read on the interconnect 22.

[0091] If the response is shared (SHD asserted, EXC deasserted), the local transaction is RdExc, and the memory bridge 32 is one of the agents reporting shared, then at least one remote node may have a shared copy of the cache block. The memory bridge 32 may initiate a probe (Kill command) to invalidate the shared copies of the cache block in the remote node(s) (reference numeral 156). In one embodiment, the data may be read from memory (or the L2 cache 36) for this case, but the transfer of the data may be delayed until the remote node(s) have acknowledged the probe. The memory bridge 32 may signal the memory controller 14/L2 cache 36 when the acknowledgements have been received. In one embodiment, each transaction may have a transaction identifier on the interconnect 22. The memory bridge 32 may transmit the transaction identifier of the RdExc transaction to the memory controller 14/L2 cache 36 to indicate that the data may be transmitted.

[0092] If the response is shared, the local transaction is RdExc, and the sharing agents are local agents (i.e. the memory bridge 32 does not report shared), then the L2 cache 36 or the memory controller 14 may supply the data, depending on whether or not there is an L2 hit for the cache block (reference numeral 158). Similarly, if the response is shared and the transaction is not RdExc, the L2 cache 36 or the memory controller 14 may supply the data dependent on whether or not there is an L2 hit for the cache block.

[0093] If the transaction is remote and uncacheable, then the memory bridge 32 may generate a noncoherent read command on the interfaces 30 to read the data. For example, a standard HT read command may be used (reference numeral 160). If the remote transaction is cacheable and the response on the interconnect 22 is exclusive, then the exclusive owner supplies the data for the read (reference numeral 162). If the remote transaction is cacheable, the response is not exclusive, the cache block is an L2 cache hit, and the transaction is either RdShd or the transaction is RdExc and the L2 cache has the block in the modified state, then the L2 cache 36 supplies the data for the read (reference numeral 164). Otherwise, the memory bridge 32 initiates a corresponding read command to the home node of the cache block (reference numeral 166).

[0094] Turning next to FIG. 8, a decision tree for a write transaction to a memory space address on the interconnect 22 of a node 10 is shown for one embodiment. The decision tree may illustrate operation of the node for the write transaction for different conditions of the transaction, the state of the cache block accessed by the transaction, etc. The write transaction may, in one embodiment, include the Wr, WrInv, and WrFlush transactions shown in the table 142 of FIG. 5. Each dot on the lines within the decision tree represents a divergence point of one or more limbs of the tree, which are labeled with the corresponding conditions. Where multiple limbs emerge from a dot, taking one limb also implies that the conditions for the other limbs are not met. In FIG. 8, the exclamation point (“!”) is used to indicate a logical NOT. Not shown in FIG. 8 is the state transition made by each coherent agent which is caching a copy of the cache block for the write transaction. The coherent agent invalidates its copy of the cache block.

[0095] If the transaction is a local transaction, and the transaction is a WrInv transaction that hits in the remote line directory 34 (i.e. a remote node is caching a copy of the cache block), the memory controller 14 (and the L2 cache 36, if an L2 hit) updates with the write data (reference numeral 170). Additionally, the memory bridge 32 may generate probes to the remote nodes indicated by the remote line directory 34. The update of the memory/L2 cache may be delayed until the probes have been completed, at which time the memory bridge 32 may transmit the transaction identifier of the WrInv transaction to the L2 cache 36/memory controller 14 to permit the update.

[0096] If the local transaction is uncacheable or if the L2 cache 36 is the master of the transaction (that is, the L2 cache 36 initiated the transaction), then the memory controller 14 updates with the data (reference numeral 172). If the local transaction is cacheable, the memory controller 14 and/or the L2 cache 36 updates with the data based on whether or not there is an L2 cache hit (and, in some embodiments, based on an L2 cache allocation indication in the transaction, which allows the source of the transaction to indicate whether or not the L2 cache allocates a cache line for an L2 cache miss) (reference numeral 174).

[0097] If the transaction is a remote transaction, the transaction is a WrFlush transaction, and the response to the transaction is exclusive, the exclusive owner supplies the data (reference numeral 176). If the remote WrFlush transaction results in a non-exclusive response (shared or invalid), the L2 cache 36 supplies the data of the WrFlush transaction. In one embodiment, the L2 cache 36 retains the state of the node as recorded in the home node, and the L2 cache 36 uses the WrFlush transaction to evict a remote cache block which is in the modified state in the node. Thus, if another agent has the cache block in the exclusive state, that agent may have a more recent copy of the cache block that should be returned to the home node. Otherwise, the L2 cache 36 supplies the block to be returned to the home node (reference numeral 182). In either case, the memory bridge 32 may capture the WrFlush transaction and data, and may perform a WB command to return the cache block to the home node.

[0098] If the remote transaction is not a WrFlush transaction, and is not cache coherent, the memory bridge 32 receives the write transaction and performs a noncoherent Wr command (e.g. a standard HT write) to transmit the cache block to the home node (reference numeral 180). If the remote transaction is not a WrFlush transaction, is cache coherent, and is an L2 hit, the L2 cache 36 may update with the data (reference numeral 182).

[0099] Turning next to FIG. 9, a block diagram illustrating operation of one embodiment of the memory bridge 32 in response to various coherency commands received from the interface circuits 20A-20C is shown. The received command is shown in an oval. Commands initiated by the memory bridge 32 in response to the received command (and the state of the affected cache block as indicated in the remote line directory 34) are shown in solid boxes. Dotted boxes are commands received by the memory bridge 32 in response to the commands transmitted in the preceding solid boxes. The cache block affected by a command is shown in parentheses after the command.

[0100] In one embodiment, the remote line directory 34 may be accessed in response to a transaction on the interconnect 22. In such an embodiment, the memory bridge 32 may initiate a transaction on the interconnect 22 in response to certain coherent commands in order to retrieve the remote line directory 34 (as well as to affect any state changes in the coherent agents coupled to the interconnect 22, if applicable). In other embodiments, the memory bridge 32 may be configured to read the remote line directory 34 prior to generating a transaction on the interconnect 22, and may conditionally generate a transaction if needed based on the state of the remote line directory 34 for the requested cache block. Additionally, in one embodiment, the remote line directory 34 may maintain the remote state for a subset of the local cache blocks that are shareable remotely (e.g. a subset of the portion of the remote coherent space 148 that is assigned to the local node). If a cache block is requested by a remote node using a coherency command and there is no entry in the remote line directory 34 for the cache block, then a victim cache block may be replaced in the remote line directory 34 (and probes may be generated to invalidate the victim cache block in remote nodes). In other embodiments, the remote line directory 34 may be configured to track the state of each cache block in the portion of the remote coherent space 148 that is assigned to the local node. In such embodiments, operations related to the victim cache blocks may be omitted from FIG. 9.

[0101] For a cRdShd command for cache block “A” received by the memory bridge 32 (reference numeral 190), the memory bridge 32 may generate a RdShd transaction on the interconnect 22. Based on the remote line directory (RLD) state for the cache block A, a number of operations may occur. If the RLD state is shared, or invalid and there is an entry available for allocation without requiring a victim cache block to be evicted (“RLD empty” in FIG. 9), then the memory bridge 32 may transmit a fill command to the remote node with the data supplied to the memory bridge 32 in response to the RdShd transaction on the interconnect 22 (reference numeral 192). On the other hand, if the RLD state is invalid and an eviction of a victim block is used to free an RLD entry for cache block A, then the memory bridge 32 may transmit probes to the remote nodes having copies of the victim cache block. If the victim cache block is shared, the memory bridge 32 may transmit a Kill command (or commands, if multiple nodes are sharing the victim cache block) for the victim block (reference numeral 194). The remote nodes respond with Kill_Ack commands for the victim block (reference numeral 196). If the victim block is modified, the memory bridge 32 may transmit a Flush command to the remote node having the modified state (reference numeral 198). The remote node may return the modified block with a WB command (reference numeral 200). In either case of evicting a victim block, the memory bridge 32 may, in parallel, generate a Fill command for the cache block A (reference numeral 92, via arrow 202). Finally, if the RLD state is modified for the cache block A, the memory bridge 32 may generate a Flush command for the cache block A to the remote node (reference numeral 204), which responds with a WB command and the cache block A (reference numeral 206). The memory bridge 32 may then transmit the Fill command with the cache block A provided via the write back command (reference numeral 192).

[0102] In response to a cRdExc command for a cache block A (reference numeral 210), operation may be similar to the cRdShd case for some RLD states. Similar to the cRdShd case, the memory bridge 32 may initiate a RdExc transaction on the interconnect 22 in response to the cRdExc command. Similar to the cRdShd case, if the RLD is invalid and no eviction of a victim cache block is needed in the RLD to allocate an entry for the cache block A, then the memory bridge 32 may supply the cache block supplied on the interconnect 22 for the RdExc transaction in a fill command to the remote node (reference numeral 212). Additionally, if the RLD state is invalid for the cache block A and a victim cache block is evicted from the RLD 34, the memory bridge 32 may operate in a similar fashion to the cRdShd case (reference numerals 214 and 216 and arrow 222 for the shared case of the victim block and reference numerals 218 and 220 and arrow 222 for the modified case of the victim block). If the RLD state is modified for the cache block A, the memory bridge 32 may operate in a similar fashion to the cRdShd case (reference numerals 224 and 226). If the RLD state is shared for the cache block A, the memory bridge 32 may generate Kill commands for each remote sharing node (reference numeral 228). The memory bridge 32 may wait for the Kill_Ack commands from the remote sharing nodes (reference numeral 230), and then transmit the Fill command with the cache block A provided on the interconnect 22 in response to the RdExc transaction (reference numeral 212).

[0103] In response to a Wr command to the cache block A, the memory bridge 32 may generate a Wr transaction on the interconnect 22 (reference numeral 240). If the RLD state is invalid for the cache block A, the memory bridge 32 may transmit the write data on the interconnect 22 and the Wr command is complete (reference numeral 242). If the RLD state is shared for the cache block A, the memory bridge 32 may generate Kill commands to each remote sharing node (reference numeral 244) and collect the Kill_Ack commands from those remote nodes (reference numeral 246) in addition to transmitting the data on the interconnect 22. If the RLD state is modified for a remote node, the memory bridge 32 may generate a Flush command to the remote node (reference numeral 248) and receive the WB command from the remote node (reference numeral 250). In one embodiment, the memory bridge 32 may delay transmitting the write data on the interconnect 22 until the WB command or Kill_Ack commands are received (although the data returned with the WB command may be dropped by the memory bridge 32).

[0104] The above commands are received by the memory bridge 32 for cache blocks for which the node 10 including the memory bridge 32 is the home node. The memory bridge 32 may also receive Flush commands or Kill commands for cache blocks for which the node 10 is a remote node. In response to a Flush command to the cache block A (reference numeral 260), the memory bridge 32 may initiate a RdInv transaction on the interconnect 22. If the local state of the cache block is modified, the memory bridge 32 may transmit a WB command to the home node, with the cache block supplied on the interconnect 22 in response to the Rdlnv transaction (reference numeral 262). If the local state of the cache block is not modified, the memory bridge 32 may not respond to the Flush command (reference numeral 264). In this case, the node may already have transmitted a WB command to the home node (e.g. in response to evicting the cache block locally). In response to a Kill command to the cache block A (reference numeral 270), the memory bridge 32 may initiate a RdKill transaction on the interconnect 22. The memory bridge 32 may respond to the Kill command with a Kill_Ack command (reference numeral 272).

[0105] In one embodiment, the memory bridge 32 may also be configured to receive a non-cacheable read (RdNC) command (e.g. corresponding to a standard HT read) (reference numeral 280). In response, the memory bridge 32 may initiate a RdShd transaction on the interconnect 22. If the RLD state is modified for the cache block including the data to be read, the memory bridge 32 may transmit a Flush command to the remote node having the modified cache block (reference numeral 282), and may receive the WB command from the remote node (reference numeral 284). Additionally, the memory bridge 32 may supply data received on the interconnect 22 in response to the RdShd transaction as a read response (RSP) to the requesting node (reference numeral 286).

[0106] Computer Accessible Medium

[0107] Turning next to FIG. 10, a block diagram of a computer accessible medium 300 including one or more data structures representative of the circuitry included in the node 10 is shown. Generally speaking, a computer accessible medium may include storage media such as magnetic or optical media, e.g., disk, CD-ROM, or DVD-ROM, volatile or non-volatile memory media such as RAM (e.g. SDRAM, RDRAM, SRAM, etc.), ROM, etc., as well as media accessible via transmission media or signals such as electrical, electromagnetic, or digital signals, conveyed via a communication medium such as a network and/or a wireless link.

[0108] Generally, the data structure(s) of the circuitry on the computer accessible medium 300 may be read by a program and used, directly or indirectly, to fabricate the hardware comprising the circuitry. For example, the data structure(s) may include one or more behavioral-level descriptions or register-transfer level (RTL) descriptions of the hardware functionality in a high level design language (HDL) such as Verilog or VHDL. The description(s) may be read by a synthesis tool which may synthesize the description to produce one or more netlist(s) comprising lists of gates from a synthesis library. The netlist(s) comprise a set of gates which also represent the functionality of the hardware comprising the circuitry. The netlist(s) may then be placed and routed to produce one or more data set(s) describing geometric shapes to be applied to masks. The masks may then be used in various semiconductor fabrication steps to produce a semiconductor circuit or circuits corresponding to the circuitry. Alternatively, the data structure(s) on computer accessible medium 300 may be the netlist(s) (with or without the synthesis library) or the data set(s), as desired. In yet another alternative, the data structures may comprise the output of a schematic program, or netlist(s) or data set(s) derived therefrom.

[0109] While computer accessible medium 300 includes a representation of the node 10, other embodiments may include a representation of any portion of the node 10 (e.g. processors 12A-12N, memory controller 14, L2 cache 36, interconnect 22, memory bridge 32, remote line directory 34, switch 18, interface circuits 22A-22C, etc.).

[0110] Numerous variations and modifications will become apparent to those skilled in the art once the above disclosure is fully appreciated. It is intended that the following claims be interpreted to embrace all such variations and modifications. 

What is claimed is:
 1. A node comprising: a first agent, a second agent, and a third agent, all coupled to an interconnect; wherein the first agent is configured to initiate a transaction on the interconnect to transfer a coherency block to the second agent; and wherein the third agent is configured to transmit the coherency block on the interconnect during a data portion of the transaction instead of the first agent responsive to a state of the coherency block in the third agent.
 2. The node as recited in claim 1 wherein the coherency block is a remote coherency block.
 3. The node as recited in claim 2 wherein the first agent is designated to retain a state of the remote coherency block within the node.
 4. The node as recited in claim 3 wherein the first agent comprises a cache, and wherein the cache is configured to initiate the transaction in response to evicting the remote coherency block.
 5. The node as recited in claim 4 wherein the cache is configured to initiate a different transaction in response to evicting a local coherency block.
 6. The node as recited in claim 2 wherein the second agent is responsible for maintaining internode coherency within the node.
 7. The node as recited in claim 1 wherein the third agent is configured to signal the first agent during a response portion of the transaction to indicate that the third agent is to transmit the coherency block during the data portion.
 8. The node as recited in claim 1 wherein the third agent is configured to transmit the coherency block if the state of the coherency block in the third agent is modified.
 9. The node as recited in claim 1 wherein the first agent is configured to transmit the coherency block on the interconnect during the data portion of the transaction responsive to the third agent not having the state of the coherency block that causes the third agent to transmit the coherency block.
 10. The node as recited in claim 1 wherein the third agent is configured to snoop the transaction from the interconnect and is configured to detect the state of the coherency block in response to snooping.
 11. A method comprising: a first agent initiating a transaction on an interconnect to transfer a coherency block to a second agent; a third agent transmitting the coherency block on the interconnect during a data portion of the transaction instead of the first agent responsive to a state of the coherency block in the third agent.
 12. The method as recited in claim 11 wherein the coherency block is a remote coherency block.
 13. The method as recited in claim 12 wherein the first agent is designated to retain a state of the remote coherency block within the node, and wherein the first agent comprises a cache, the method further comprising the cache evicting the remote coherent block, and wherein the first agent initiating the transaction is in response to the evicting the remote coherency block.
 14. The method as recited in claim 13 further comprising the cache initiating a different transaction in response to evicting a local coherency block.
 15. The method as recited in claim 12 wherein the second agent is responsible for maintaining internode coherency within the node.
 16. The method as recited in claim 11 wherein the state of the coherency block in the third agent is modified.
 17. The method as recited in claim 11 further comprising the first agent transmitting the coherency block on the interconnect during the data portion of the transaction responsive to the state in the third agent not causing the third agent to transmit the coherency block.
 18. The method as recited in claim 11 further comprising the third agent snooping the transaction from the interconnect, wherein the third agent transmitting the coherency block is responsive to the snooping.
 19. A node comprising: an interconnect; and a cache coupled to the interconnect, wherein the cache is configured to evict a cache block stored therein, and wherein the cache is configured to initiate a first transaction to transfer the cache block on the interconnect responsive to the cache block being a local cache block, and wherein the cache is configured to initiate a second transaction to transfer the cache block on the interconnect responsive to the cache block being a remote cache block, and wherein the second transaction is different from the first transaction.
 20. The node as recited in claim 19 further comprising a memory bridge coupled to the interconnect and responsible for internode coherency within the node, wherein the memory bridge is configured to receive the second transaction and to transfer the cache block to another node.
 21. The node as recited in claim 20 further comprising a memory controller coupled to the interconnect and configured to receive the first transaction, wherein the memory controller is configured to transfer the cache block to a memory to which the memory controller is configured to be coupled.
 22. The node as recited in claim 19 further comprising an agent coupled to the interconnect, wherein the agent is configured to transmit the cache block in a data portion of the second transaction instead of the cache responsive to the agent having a modified copy of the cache block. 